Crate chainrules_core 

chainrules-core

chainrules-core defines the engine-independent AD protocol used by chainrules-rs.

It is intentionally small and does not provide function rules. The crate exists to define the traits and error types that downstream AD engines and rule libraries build on.

What It Provides

• Differentiable
• ReverseRule
• ForwardRule
• AutodiffError
• NodeId
• SavePolicy

Example

use chainrules_core::NodeId;

let id = NodeId::new(7);
assert_eq!(id.index(), 7);

Notes

This crate is protocol-only. Shared scalar frule and rrule helpers live in chainrules. Core AD trait definitions (like Julia’s ChainRulesCore.jl).

This crate defines the interface for automatic differentiation without providing an AD engine. It contains:

• Differentiable — tangent space definition for any value type
• ReverseRule — per-operation reverse-mode rule (rrule/pullback)
• ForwardRule — per-operation forward-mode rule (frule/pushforward)
• Error types (AutodiffError, AdResult)
• NodeId, SavePolicy — graph node identifier and save strategy

AD engines (Tape, TrackedValue, DualValue, pullback, hvp) live in separate crates, for example tidu.

Operation-specific AD rules (e.g., einsum rrule/frule) live in the crate that defines the operation.

Examples

Implementing Differentiable for a custom type:

use chainrules_core::Differentiable;

#[derive(Clone)]
struct MyVec(Vec<f64>);

impl Differentiable for MyVec {
    type Tangent = MyVec;
    fn zero_tangent(&self) -> MyVec {
        MyVec(vec![0.0; self.0.len()])
    }
    fn accumulate_tangent(a: MyVec, b: &MyVec) -> MyVec {
        MyVec(a.0.iter().zip(&b.0).map(|(x, y)| x + y).collect())
    }
    fn num_elements(&self) -> usize {
        self.0.len()
    }
    fn seed_cotangent(&self) -> MyVec {
        MyVec(vec![1.0; self.0.len()])
    }
}

Structs

NodeId

Stable identifier of an AD graph node.

Enums

AutodiffError

AD-specific error type.

SavePolicy

Saved-tensor retention policy for reverse-mode rules.

Traits

Differentiable

Trait defining the tangent space for a differentiable type.

ForwardRule

Forward-mode AD rule interface (frule).

ReverseRule

Reverse-mode AD rule interface (rrule).

Type Aliases

AdResult

Result alias for AD APIs.

PullbackEntry

Reverse-rule pullback output entry (input_node, input_cotangent).

PullbackWithTangentsEntry

Reverse-rule pullback-with-tangents output entry.